1. Field of the Invention
The present invention relates generally to a speech recognition system for an automotive vehicle, and more particularly to a speech recognition system by which driver's spoken instructions can be reliably recognized even as noise fluctuates intensely within the passenger compartment.
2. Description of the Prior Art
There is a well-known speech recognizer which can activate various actuators in response to human spoken instructions. When this speech recognizer is mounted on a vehicle, the headlight, for instance, can be turned on or off in response to spoken instructions such as "Headlight on" or "Headlight off". Such a speech recognizer usually can recognize various spoken instructions in order to control various actuators; however, there are some problems involved in applying this system to an automotive vehicle.
Usually, the speech recognizer is used in a relatively quiet environment; however, the speech recognition system for an automotive vehicle is usually used within a relatively noisy passenger compartment and additionally the noise fluctuates intensely therewithin, in particular, when the vehicle windows are kept opened and when the vehicle is running on a noise city street. Therefore, one of the major problems is how to cope with erroneous spoken phrase recognitions caused by fluctuating noise within the passenger compartment.
In order to distinguish a spoken instruction from noise, conventionally there is provided a voice detector in the speech recognizer, by which the start and the end of a spoken instruction are determined by detecting whether the magnitude of a spoken instruction signal exceeds a predetermined reference threshold voltage level for a predetermined period of time or whether the magnitude of the spoken instruction signal drops below the predetermined reference threshold voltage level for another predetermined period of time, respectively.
By the way, a person or driver has a tendency to speak quickly and loudy when the background noise level is relatively high but slowly and in a whisper when the background noise level is relatively low. Therefore, it is necessary to provide a wide dynamic range for the speech recognizer. Here, the dynamic range means the ratio of the loudest to the weakest sound intensity which can be detected by the system.
In the prior-art speech recognizer, however, since analog-digital converters are usually incorporated within the speech recognizer, it is very difficult to provide a sufficiently wide dynamic range which can cover a wide sound intensity from a loud voice to a low voice.
In order to overcome the above-mentioned problems, there has been proposed a method of incorporating a gain controller at the input stage of the speech recognizer. In this case, the gain of the recognizer is adjusted inversely proportional to the sound intensity inputted thereto, so that the sound levels of spoken instructions can automatically be adjusted within a narrower range.
In such a prior-art speech recognizer as described above, however, in the case where a spoken instruction is uttered toward the microphone immediately after a loud noise such as horn sound has been produced, since the gain of the gain controller has already been adjusted to a lower level, there exists a problem in that it is impossible to reliably detect the start of a spoken instruction.
A more detailed description of a typical prior-art speech recognizer will be made with reference to the attached drawings in conjunction with the present invention under DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.